Diagnostic assays are widespread and central for the diagnosis, treatment and management of many diseases. Different types of diagnostic assays have been developed over the years in order to simplify the detection of various analytes in clinical samples such as blood, serum, plasma, urine, saliva, tissue biopsies, stool, sputum, skin or throat swabs and tissue samples or processed tissue samples. These assays are frequently expected to give a fast and reliable result, while being easy to use and inexpensive to manufacture. Understandably it is difficult to meet all these requirements in one and the same assay. In practice, many assays are limited by their speed. Another important parameter is sensitivity. Recent developments in assay technology have led to increasingly more sensitive tests that allow detection of an analyte in trace quantities as well the detection of disease indicators in a sample at the earliest time possible.
A common type of disposable assay device includes a zone or area for receiving the liquid sample, a conjugate zone also known as a reagent zone, and a reaction zone also known as a detection zone. These assay devices are commonly known as lateral flow test strips. They employ a porous material, e.g., nitrocellulose, defining a path for fluid flow capable of supporting capillary flow. Examples include those shown in U.S. Pat. Nos. 5,559,041, 5,714,389, 5,120,643, and 6,228,660 all of which are incorporated herein by reference in their entireties.
The sample-addition zone frequently consists of a more porous material, capable of absorbing the sample, and, when separation of blood cells is desired, also effective to trap the red blood cells. Examples of such materials are fibrous materials, such as paper, fleece, gel or tissue, comprising e.g. cellulose, wool, glass fiber, asbestos, synthetic fibers, polymers, or mixtures of the same.
Another type of assay device is a non-porous assay having projections to induce capillary flow. Examples of such assay devices include the open lateral flow device as disclosed in WO 2003/103835, WO 2005/089082, WO 2005/118139, and WO 2006/137785, all of which are incorporated herein by reference in their entireties.
A known non-porous assay device is shown in FIG. 1. The assay device 1, has at least one sample addition zone 2, a reagent zone 3, at least one detection zone 4, and at least one wicking zone 5. The zones form a flow path by which sample flows from the sample addition zone to the wicking zone. Also included are capture elements, such as antibodies, in the detection zone 4, capable of binding to the analyte, optionally deposited on the device (such as by coating); and a labeled conjugate material also capable of participating in reactions that will enable determination of the concentration of the analyte, deposited on the device in the reagent zone, wherein the labeled conjugate material carries a label for detection in the detection zone. The conjugate material is dissolved as the sample flows through the reagent zone forming a conjugate plume of dissolved labeled conjugate material and sample that flows downstream to the detection zone. As the conjugate plume flows into the detection zone, the conjugated material will be captured by the capture elements such as via a complex of conjugated material and analyte (as in a “sandwich” assay) or directly (as in a “competitive” assay). Unbound dissolved conjugate material will be swept past the detection zone into the at least one wicking zone 5.
An instrument such as that disclosed US 20060289787A1, US20070231883A1, U.S. Pat. Nos. 7,416,700 and 6,139,800 all incorporated by reference in their entireties, is able to detect the bound conjugated material in the detection zone. Common labels include fluorescent dyes that can be detected by instruments which excite the fluorescent dyes and incorporate a detector capable of detecting the fluorescent dyes.
The sample size for such typical assay devices as shown in FIG. 1 are generally on the order of 200 μl. Such a sample size requires a venous blood draw from a medical professional such as a phlebotomist. There is an increasing need for lateral flow devices that are able to function with a much smaller sample size to accommodate the amount of blood available from a so-called “fingerstick” blood draw, which is on the order of 25 μl or less. Such a small amount of sample is the amount of blood in a drop of blood after pricking a finger tip with a lancet. Home blood glucose meters typically use a drop of blood obtained in such a fashion to provide glucose levels in blood. Such a smaller sample size would not require a medical professional to draw the blood and would provide greater comfort to the patients providing the sample for analysis.
To reduce the sample size required, the dimensions of the lateral flow assay devices are reduced to accommodate the smaller sample size. However, it has been found that reducing the sample size and dimensions of the device provides inadequate conjugate in the detection zone and accordingly less signal that can be read by the instrument. The inadequate conjugate in the detection zone is believed to be due to reduced sample size and inefficient use of the sample in the device, amongst other conditions. Another drawback of reducing dimensions is that the width of the detection zone will also be reduced, again making less signal available that can be read by the instrument.
Another disadvantage with a typical assay design shown in FIG. 1 is that the length of the detection zone is very short and can only measure one analyte and cannot measure additional analytes or controls (e.g., internal positive and negative controls). While it is possible to increase the length of the detection zone along a straight line, this leads to an assay device that is larger than desired for point-of-care applications, has increased use of materials, and is more expensive to manufacture.
To gain the advantages of a longer detection zone in a smaller foot print, the detection zone can be lengthened by bending or folding the flow path of the detection zone or other part of the flow path around one or more corners to create a serpentine design that can be contained within a smaller foot print. U.S. Pat. No. 7,524,464/Publication Nos. 2010/0167318, 2009/0130658, 2009/0123336 all disclose fluidic devices having folded or serpentine flow paths.
The present inventors found, however, that placing turns or corners in the flow path of an assay device that uses micropillars or projections, will not provide satisfactory results. This is believed due to a flow rate that is slower in the outer edge of the channel (longer flow path) than the flow rate in the inner edge (shorter flow path) around the turn or corner. This leads to a reagent plume coming from the reagent zone that does not adequately spread across as much of the width of the detection zone as possible, which in turn leads to a decreased signal that can be read by the instrument reading the signal. The problems of a reagent plume not covering as much of the detection zone as possible is a particular problem in smaller devices that have narrower detection zones. In other words, it is important for the reagent plume to spread across as much of the width of the detection zone as possible to provide the maximum amount of signal to be read by the read window of the instrument. Another problem for the biased flow is that wash efficiency is poor since part of the plume near the outer edge of the turn takes much longer to get washed out due to its slower flow rate relative to the inner edge.
Accordingly, there is a need for an assay device that can provide a longer detection zone in a small footprint while maintaining desired flow characteristics of the conjugated sample through the detection zone.